This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This study aims to evaluate the safety and efficacy of a 60 mg dose of Denufosol (INS37217) Inhalation Solution compared to a placebo administered 3 times daily over 24 weeks in subjects with CF.